How do I make sure everything in my scene is in proportion to my character?
This is a very common problem in (architectural) modeling.
I'll answer it as objectively as I can.

To get a good sense of real world scale:

Universal tips  


Learn the size of everyday objects.
Get up and manually measure your windows, doors, ceiling height, etc.
You'll find that nothing is arbitrary, everything has a purpose.
Model by blueprints
Nothing beats physical experience, but blueprints and floor plans help to save time considerably, when modeling extra large/tiny objects.
Get a reference source.
Architects are one step ahead of you there.  Neufert's Architects' Data is a reference book for spatial requirements in architecture. From optimal height of door handles, to minimal size of toilet bowls /s. 


Blender tips


Model in real world scale.
Set Blender to a unit system that you use in everyday life.
Properties panel > Scene > Units  





Use orthographic view - this will allow for a more precise modeling
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